Because a selenium-accumulating plant, Neptunia amplexicaulis, excludes selenium from its proteins, it has been suggested that altered aminoacyl-tRNA synthetases from accumulators are unable to utilize the selenium analogs of the normal substrates, methionine and cysteine. Therefore, to test this hypothesis, plant species (mainly from the genus Astragalus) known to accumulate selenium from seleniferous soils will be treated with Se-75-labeled selenium compounds and examined to see if they also exclude selenium from their proteins. Methionyl-tRNA synthetases will be isolated and purified from accumulator species and from related species that not only are unable to accumulate the element but are inhibited by traces of selenium compounds. Methionine and selenomethionine will be tested to determine if methionyl-tRNA synthetases from accumulator species differ in their substrate specificities from those of nonaccummulator species. To characterize the synthetases from the two types of plants, Km, Ki and Vmax values will be calculated from Michaelis-Menton kinetic data. Heat stabilities will be studied as well as the influence of substrate and analog on heat inactivation of the synthetases.